Fix some 64 bit warnings on mac.

Fix some 64 bit warnings on mac.

SkAAClip:
Change fDataSize to a size_t, since that is how it is used in all
cases.

SkAlphaRuns.cpp:
Use SkToS32 when subtracting a pointer from another pointer to
return an int.

BUG=http://code.google.com/p/skia/issues/detail?id=1103

Committed: https://code.google.com/p/skia/source/detail?r=7591

[reed1, 2013-02-05 18:38:37]

lgtm